During the measurement of propagation time by means of ultrasonic sound, an ultrasonic wave is emitted by a transmitter, passes through an indeterminate distance and subsequently impinges on the receiver. If the propagation speed and the distance from an object are known, the propagation time of the wave can be determined. Since the transmitter (e.g., an ultrasonic transducer) is a transmitter-receiver combination, the emitted sound wave must be reflected in order to be able to be sensed again. Reflectors arranged in the sound path are used for this.
If the measurement system is composed of different materials, however, multiple problems arise. As a result of different coefficients of thermal expansion of the materials, the expansion due to a changing temperature is unequal at multiple points. This brings about a change in the measurement distance and therefore falsifies the reference distance. If, for example, a reflector made of stainless steel is attached to a plastic housing, when there is a change in the temperature the distance between the transmitter-receiver combination and the reflector will change. Furthermore, satisfactory functionality of the system cannot be ensured as a result of aging of the components. If expansion, rotation, stresses, or other wear of the individual components occur at individual points over the service life of the mechanical structure, adverse effects on the measuring unit may result. In order to limit these effects, a measurement is carried out with two reflectors (or references). In this context it is to be noted that the two reflector faces or reference faces are composed of the same material or same permanently connected parts in order to limit the mentioned effects such as heat, expansion, or aging to a single material. The propagation time of the distance between the transmitter-reflector-receiver is no longer measured here but instead the difference in propagation time between the two reflector faces (reference faces) which are different distances away.
A known ultrasonic sensor is located in a closed container (tank) which is filled with liquid. The object of the sensor is to determine the concentration of the liquid by means of a propagation time measurement. Furthermore, the sensor may measure the current filling level. In this context, the mechanical structure is arranged in such a way that a part of the sound wave which is output by a transmitter/receiver is reflected at a reference structure with two reflectors. Another part of the wave is directed to the surface of the liquid via a mirror, in order to determine the filling level.
The reference structure is arranged here in a horizontal (planar) fashion, wherein the two reflectors are formed by planar faces of the common reference structure which are at a different distance from the transmitter/receiver. This horizontal reference structure has the advantage that it is not located directly in the sound path, with the result that an excessively large part of the signal is not screened and therefore a filling level measurement is possible.
Since the overall height of the reference structure of such an ultrasonic sensor is relatively small, the risk of the accumulation of dirt which can lead to measurement falsifications is very large. Fabrication tolerances of the ultrasonic transducers can give rise to different emission angles of the sound beams and also act to a relatively pronounced degree on the measuring accuracy in the case of relatively flat reference structures which are embodied in such a way. Finally, with such reference structures with planar reference faces there is the following problem: when the container is refueled, air bubbles are also swept in or come about and become lodged at various components, which can cause a measurement to be falsified or entirely prevented. In the case of a rising temperature, gas bubbles can also form in the system as a result of out gassing if it has not yet happened as a result of the refueling process. Such bubbles can impede the measurement system, in particular can cause multiple reflections or undesired signal deflections which can falsify the measurement result.
A liquid tank is known from DE 15 48 930 A. In this publication, an ultrasonic device for measuring the level of a liquid is described. The ultrasonic wave path runs perpendicularly with respect to the level of the liquid here, and the reflectors are arranged in the form of horizontal cylinders.
U.S. Pat. No. 6,360,599 B1 discloses a device and a method for measuring a liquid level. In this device, the ultrasonic sound path also runs perpendicularly with respect to the level of the liquid, and cylindrical reflectors which are arranged in a horizontal fashion are provided.
DE 10 2006 017 284 A1 relates to a device for measuring the filling level of a liquid in a pipe with an ultrasonic sensor which is arranged on the floor of a liquid tank. A planar reflector is used here.
The device which is described in WO 2009/074428 A1 and which has the purpose of measuring a filling level of a liquid in a container has an ultrasonic sensor which is arranged on the floor of the container and has corresponding sound-conducting bodies which are embodied in a tube shape.